The sales tax issue in the US is breathtaking complicated. We did an ecommerce site for a local chain that has retail locations in a few cities round here. Colorado effectively only applies sales tax if both the consumer and the business share a jurisdiction.
So that means if both and in denver then you need to collect Denver City, Denver County, Colorado State, RTD transit and a few other miscellaneous development levies. However if the business is in denver but the consumer is up in boulder, then you only need to collect Colorado State & RTD.
Unfortunately the boundaries for the different districts are absurdly complicated. I live in a small town in colorado with a single zip code and yet we have 4 different tax rates. The transit tax only applies if you are in a part of the town that was annexed after some year. I can't get the right taxes on something delivered from the other side of town.
I'm sure any sales tax offering will start with just state level taxes being collected, but counties and cities will be right behind in wanting their share. At that point Amazon will have to maintain an accounting relationship with thousands of entities, and they'll have their unfair advantage back since it'll quickly become impractical for all but the largest online retailers to play.